Since there was no evidence found of pre-impact mechanical failure, the analysis will focus on the operational aspects of this occurrence. The loss of directional control experienced by the pilot could have been the result of a self-generated loss of tail rotor authority, or the same effect may have been caused by encountering drifting main rotor vortices. While filming the Bell 205 picking up water with a bucket, the pilot of the Bell 206B was flying a slow, parallel course in a westerly direction, low over the trees on the right side. As the Bell 205 climbed away, it flew above and ahead of the Bell 206B. Since the wind was from the southwest, conditions were favourable for the main rotor vortices from the Bell 205 to drift into the path of the Bell 206B. These vortices could cause a sudden reduction in tail rotor thrust with a resultant uncommanded right yaw that could develop into a high rate of turn. The pilot's low height over the trees precluded the reduction of collective pitch to effect recovery. A comparison of the facial injuries experienced by the pilot and the lack of injury to the forestry officer sitting in the left front seat following ground impact, leads the Board to conclude that use of a shoulder harness would have reduced the pilot's upper body injuries. The following Engineering Branch report was completed: LP 63/98 - Tail Rotor Drive Shaft ExaminationAnalysis Since there was no evidence found of pre-impact mechanical failure, the analysis will focus on the operational aspects of this occurrence. The loss of directional control experienced by the pilot could have been the result of a self-generated loss of tail rotor authority, or the same effect may have been caused by encountering drifting main rotor vortices. While filming the Bell 205 picking up water with a bucket, the pilot of the Bell 206B was flying a slow, parallel course in a westerly direction, low over the trees on the right side. As the Bell 205 climbed away, it flew above and ahead of the Bell 206B. Since the wind was from the southwest, conditions were favourable for the main rotor vortices from the Bell 205 to drift into the path of the Bell 206B. These vortices could cause a sudden reduction in tail rotor thrust with a resultant uncommanded right yaw that could develop into a high rate of turn. The pilot's low height over the trees precluded the reduction of collective pitch to effect recovery. A comparison of the facial injuries experienced by the pilot and the lack of injury to the forestry officer sitting in the left front seat following ground impact, leads the Board to conclude that use of a shoulder harness would have reduced the pilot's upper body injuries. The following Engineering Branch report was completed: LP 63/98 - Tail Rotor Drive Shaft Examination The pilot was qualified and certified for the flight in accordance with existing regulations. Records indicate that the helicopter was certified, equipped, and maintained in accordance with existing regulations and approved procedures. The pilot lost directional control and was unable to recover. There were no pre-impact mechanical failures found. The wind conditions and the helicopter's position and speed were favourable for a loss of tail rotor authority condition to develop. The pilot was not wearing the available shoulder harness, and suffered facial injuries.Findings The pilot was qualified and certified for the flight in accordance with existing regulations. Records indicate that the helicopter was certified, equipped, and maintained in accordance with existing regulations and approved procedures. The pilot lost directional control and was unable to recover. There were no pre-impact mechanical failures found. The wind conditions and the helicopter's position and speed were favourable for a loss of tail rotor authority condition to develop. The pilot was not wearing the available shoulder harness, and suffered facial injuries. It is likely that the loss of directional control experienced by the Bell 206B helicopter pilot resulted from a loss of tail rotor authority when he encountered main rotor vortices of the Bell 205 helicopter.Causes and Contributing Factors It is likely that the loss of directional control experienced by the Bell 206B helicopter pilot resulted from a loss of tail rotor authority when he encountered main rotor vortices of the Bell 205 helicopter.